Distributed Propulsion Aircraft Research Articles

Predicting noise for full-scale distributed propulsion aircraft by synthesizing subscale data with existing prediction tools

Future projections of the economic impact of Urban Air Mobility (UAM) or Advanced Air Mobility (AAM) are bold with global GDP increases ranging from billions to trillions over the next few decades. While the potential opportunities are vast, there will be a paradigm shift in the sheer volume of flights at lower altitudes near populated communities. While there is still debate on the initial markets, there is a strong consensus that these aircraft must not appreciably contribute to the existing soundscape and ideally would be imperceptible in most cases. The novel designs of many distributed propulsion aircraft are challenging the existing noise prediction tools that were primarily built around propellers, jet aircraft, and rotorcraft. This paper will present a summary of recent research efforts on synthesizing subscale experimental acoustics measurement data with noise propagation tools to compute community noise exposure on the ground for different full-scale vehicle configurations and flight profiles. The benefits, drawbacks, and limitations of each data source will be discussed along with suggestions for how to improve the prediction methods and models by synthesizing multiple data sets.

A Jacobi-Ritz Approach for Aeroelastic Analysis of Swept Distributed Propulsion Aircraft Wing

Abstract This paper presents a Jacobi-Ritz approach for conducting flutter and divergence analysis of a complex distributed propulsion aircraft wing like NASA X-57. The general orthogonal Jacobi polynomials are used to approximate the bending displacement and torsional rotation angle in the Ritz method based structural and aeroelastic analysis. The Jacobi polynomials satisfy the orthogonality condition using weight functions which are easily modified to satisfy different essential and natural boundary conditions. Compared to simple polynomials, Jacobi polynomials can eliminate the well-known ill-conditioning numerical issues when considering higher-order polynomials. The Jacobi-Ritz method is also found to alleviate mode switching often encountered in tracking the changes of modes with the varying airspeed. The Jacobi-Ritz method is later used to investigate the flutter and divergence speeds under distributed propulsor mass and their locations, nonuniform aerodynamic model in the presence of multiple propulsors, and the sweep angle. Results show that placing the distributed propulsors on the wing's leading edge increases the flutter speed even though the bending and torsion modal frequencies are decreased compared to those of the wing without propulsors. The presence of pods for the middle high-lift motors causes an extra aerodynamic moment which reduces the flutter speed. Parametric studies also show that the divergence speed is lower than the flutter speed for a uniform and straight distributed propulsor wing. Using swept-back wing configuration and placing the tip propulsor near the wing's leading edge can help to increase both flutter and divergence speeds.

Feasibility Study on Copper-Clad Lithium Conductors for Cryogenic Applications

The trend towards efficient turbo-electric distributed propulsion aircraft calls for more research concerning lightweight power distribution and power conversion, especially in the context of cryogenic power systems. This paper explores one such avenue in this context – the prospect of manufacturing and using copper-clad lithium (CuCLi) wires at cryogenic conditions for aircraft and high frequency applications – and compares it to Yttrium Barium Copper Oxide (YBCO) coated superconductors. Lithium, a lightweight and inexpensive metal, exhibits superior conductivity per unit weight characteristics compared to copper, aluminum, and other metals at cryogenic temperatures. While the metal always remains in its resistive hence lossy state, it is unlikely to reach the same current densities as superconductors. On the other hand, copper-clad lithium wires are expected to be less expensive, are round, and can be manufactured at long lengths and in large quantities. In this paper, the relevant electric parameters of these wires, such as current carrying capability and power losses, are estimated and compared to those of superconductors. The paper also includes a brief study of the manufacturing method of proposed copper-clad lithium wires.

Sizing of Superconducting Cables for Turbo-ElectricDistributed Propulsion Aircraft Using a Particle Swarm Optimization Approach

Superconducting electrical power systems are proposed to meet high specific power densities required for turbo-electric distributed propulsion aircraft. Superconducting materials have unique thermal and electrical requirements for maintaining the superconducting state, which is critical to their normal operation. Electrical system faults can lead to this state being lost for all network assets in the electrical fault path. The resulting temperature rise can prevent the superconducting state from being immediately resumed following fault clearance, requiring disconnection of nonfaulted equipment. Undersized cables experience a higher temperature rise under faulted conditions and disconnect from the system more readily. Oversized cables are heavier and more costly. Therefore, there is a need to optimize the cable size, preventing disconnection of equipment due to temperature rise following a fault while minimizing the weight and cost penalty. This article proposes a system parameter-driven methodology, using particle swarm optimization, to identify fault-tolerant cable designs, which deliver minimum through-life costs. This facilitates high-value, quantifiable design trade studies incorporating system parameters. Key observations drawn are that the choice between improving fault ride-through capability of a superconducting cable by increasing the amount of either superconducting material or conventional former material strongly depends on acceptable system operating temperature and voltage.

Design Space Exploration of Lithium-Ion Battery Packs for Hybrid-Electric Regional Aircraft Applications

Distributed hybrid and electric propulsion systems are one of the most promising technologies to reduce aircraft emissions, resulting in research efforts to investigate new architectures and the design of optimal energy management strategies. This work defines the optimal requirements in terms of battery pack sizing and cell technology for a hybrid-electric regional wing-mounted distributed propulsion aircraft through the application of a design space exploration method. The propulsion system considered in this study is a series-parallel hybrid turboelectric power train with distributed electric fans. A set of six lithium-ion battery cell technologies was identified and experimentally characterized, including both commercially available and prototype cells at different combinations of specific energy and power. A model of the aircraft was developed and used to define the optimal energy management strategy for the hybrid turboelectric propulsion system, which was solved using dynamic programming. The design space exploration was conducted by varying the cell technology and battery storage system size; and the effects on fuel consumption, energy management strategy, and thermal management were compared.

Flight dynamics and control assessment for differential thrust aircraft in engine inoperative conditions including aero-propulsive effects

Differential thrust can be used for directional control on distributed electric propulsion aircraft. This paper presents an assessment of flight dynamics and control under engine inoperative conditions at minimum control speed for a typical distributed propulsion aircraft employing differential thrust. A methodology consisting of an aerodynamic data acquisition module and a non-linear six-degrees-of-freedom flight dynamics model is proposed. Directional control is achieved using a controller to generate a yaw command, which is distributed to the propulsors through a thrust mapping approach. A modified version of the NASA X-57 aircraft is selected for case studies, where the engine inoperative condition is considered to impact the three leftmost propulsors during climb at minimum control speed. The objective also includes the assessment of the impact of the aero-propulsive coupling for such an aircraft during a failure case. Results show that during the recovery manoeuvre, the aircraft experiences a 78% reduction in total thrust and 30% reduction in total lift caused by the aggressive yaw control effort required to control the heading of the aircraft. Consequently, the powered-stall speed is increased, and the aircraft temporarily loses altitude during the recovery manoeuvre. Differential thrust provides sufficient yaw authority during the engine inoperative condition, and is, therefore, seen to potentially replace the functionality of the rudder for the climb condition that was studied. Additionally, reduction of the vertical tail area was explored and seen to be possible if the response time of the system is low enough. For the studied configuration, this required a response within 400 ms for reduced vertical tail areas.

Aero-propulsive interaction model for conceptual distributed propulsion aircraft design

PurposeThis research aims to present an aero-propulsive interaction model applied to conceptual aircraft design with distributed electric propulsion (DeP). The developed model includes a series of electric ducted fans integrated into the wing upper trailing edge, taking into account the effect of boundary layer ingestion (BLI). The developed model aims to estimate the aerodynamic performance of the wing with DeP using an accurate low-order computational model, which can be easily used in the overall aircraft design's optimization process.Design/methodology/approachFirst, the ducted fan aerodynamic performance is investigated using a low-order computational model over a range of angle of attack required for conventional flight based on ducted fan design code program and analytical models. Subsequently, the aero-propulsive coupling with the wing is introduced. The DeP location chordwise is placed at the wing's trailing edge to have the full benefits of the BLI. After that, the propulsion integration process is introduced. The nacelle design's primary function is to minimize the losses due to distortion. Finally, the aerodynamic forces of the overall configuration are estimated based on Athena Vortex Lattice program and the developed ducted fan model.FindingsThe ducted fan model is validated with experimental measurements from the literature. Subsequently, the overall model, the wing with DeP, is validated with experimental measurements and computational fluid dynamics, both from the literature. The results reveal that the currently developed model successfully estimates the aerodynamic performance of DeP located at the wing trailing edge.Originality/valueThe developed model's value is to capture the aero-propulsive coupling accurately and fast enough to execute multiple times in the overall aircraft design's optimization loop without increasing runtime substantially.

Effect of Arc Chute on DC Current Interruption by Liquid Nitrogen in HTS Electrical System of Distributed Propulsion Aircraft

The distributed propulsion aircraft with HTS electrical system is a novel concept for future airliners, which can reduce by more than 70% fuel burn and NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> emissions. The circuit breakers ensure the security of this novel aircraft by isolating electrical faults timely. Solid-state circuit breakers (SSCBs) are preferred due to their fast response and high performance in the cryogenic circumstance. However, the high conduction loss of SSCBs impedes their further application. A mechanical switch using liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) as an arc extinguishing medium shows excellent DC current interruption performance. The LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> switch is characterized with extremely low contact resistance, and the proper use may reduce the conduction loss of power switches significantly. Nevertheless, the effect of metal type arc chutes on the arcing process in the LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is still not clear. Thus the objective of this paper is to understand the effect of metal type arc chutes on the current interruption performance of LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . Silicon iron arc chutes are employed. Neodymium (NdFeB) magnets are used to stretch the arc into the arc chutes. The maximum interrupting current is 1 kV/ 2 kA when only magnets are applied. Further applying the arc chutes leads to a significant drop in the arc voltage and interruption performance. Since the high relative permeability of silicon iron weakens the magnetic field acting on the arc, metal type arc chutes are not recommended. 1 kV / 10 kA fault current is successfully cleared by the combination of resistance type superconducting fault current limiter (R-SFCL) and LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> switch with magnets, during which the R-SFCL responds to the fault within 420 μs, compensating the long clear time of the LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> switch.

Metal composite T-junction terminals for MW-class aerospace electric power distribution

There is a recent surge in activity to develop high power electric (or hybrid electric) aircraft. Part of this development effort is the creation of lightweight and small volume high-performance motors and airborne power transmission cables. As part of the power transmission of a distributed propulsion aircraft will be T-terminals to extract power to individual motors from a “main” power cable. In this research, a standard pressed plate high purity Cu T-terminal, with cylindrical high-temperature superconducting cables (main cable current of 20 kA, branch cable current of 2.5 kA), were investigated using Multiphysics simulations. Then, a more geometrically optimized high purity Al-Cu composite T-terminal was simulated under similar conditions. Discussed are the influence of T-junction geometry, operating temperature (30 to 50 K), contact resistance, and magnetoresistance on joule losses of terminals with different masses. It is shown the Al-Cu terminal can greatly reduce joule losses/mass of the T-terminal while also having an intrinsic clamping force from thermal expansion of the Al shell of the composite structure.

A parallel, object-oriented framework for unsteady free-wake analysis of multi-rotor/wing systems

The development, validation, and applications of an object-oriented free-wake solver for multi-rotor and fixed-wing systems are outlined here. Advantages of utilizing the object-oriented philosophy for modeling the multi-rotor/wing free-wake problem are described. To explore the feasibility of utilizing conventional desktop workstations, the vortex lattice methodology’s time complexity is examined as an evolving n-body problem and the advantage gained from employing wake roll up models is demonstrated. Modifications in typical formulae to leverage features of multi-core systems and improvements in computational performance obtained from these modifications are illustrated using a roofline analysis. The free-wake solver incorporating these optimizations is then employed—after extensive validation—to simulate a distributed propulsion aircraft and a multi-rotor urban mobility vehicle. The paper also proposes a novel lattice skew parameter for monitoring instabilities in wake solutions commonly observed in rotorcraft free-wake analysis.

Actuator 기법을 이용한 프로펠러 3축 성분 힘/모멘트 해석

For aerodynamic performance and stability analysis of distributed propulsion aircraft which adopt many propellers/rotors, all the 3-axial components forces and moments must be properly taken into account. In this study, two actuator methods have been established to be capable of numerical simulations including all the force and moment components produced by rotor/propeller. A procedure in which the 3-axial components are transformed from the aerodynamic force and moment of blade elements has been added to actuator disk method(ADM) and actuator surface method(ASM). The numerical analyses are carried out for a single propeller of quad tilt propeller(QTP) at various flight conditions including the variation of side slip angle. The results are compared with wind tunnel test data and full CFD simulation with overset grid. All the force and moment components are found to agree very well in terms of engineering purposes and the present methods are identified to be numerically very efficient compared to full CFD. Power-on effect and longitudinal stability of the full configuration of QTP is further analyzed by carrying out the additional numerical simulations. The results indicate that the present methods is practically very useful to investigate the aerodynamic performance of distributed propulsion aircraft.

Distributed Propulsion Aircraft with Aeroelastic Wing Shaping Control for Improved Aerodynamic Efficiency

This study presents an aeroelastic wing shaping control concept for distributed propulsion aircraft. By leveraging wing flexibility, wing-mounted distributed propulsion can be used to re-twist wing shapes in-flight to improve aerodynamic efficiency. A multidisciplinary approach is used to develop an aero-propulsive-elastic model of a highly flexible wing distributed propulsion transport aircraft. The conceptual model is used to evaluate the aerodynamic benefit of the distributed propulsion aircraft. The initial conceptual analysis shows that an improvement in the aerodynamic efficiency quantity of lift-to-drag ratio is possible with the proposed aeroelastic wing shaping control for distributed propulsion aircraft. Two concepts are studied: single-generator configuration and dual-generator configuration with four propulsors per wing. The baseline aircraft model is NASA Generic Transport Model. A fan performance analysis is developed for propulsion sizing. Cruise performance analysis is conducted to evaluate the potential improvement in the cruise range for the configurations under study. A flutter analysis is performed to address the potential flutter issue as the propulsors are placed toward the wing tip, which would cause a reduction in the wing natural frequencies. Flight control considerations are addressed in the context of the engine-out requirement, yaw and roll controls, and yaw damping augmentation using differential thrust.

Comparison of Candidate Architectures for Future Distributed Propulsion Aircraft

Turbine-engine-driven distributed electrical aircraft power systems [also referred to as turboelectric distributed propulsion (TeDP)] are proposed for providing thrust for future aircraft with superconducting components operating at 77 K, in order for performance and emission targets to be met. The proposal of such systems presents a radical change from current state-of-the-art aeroelectrical power systems. Central to the development of such power systems are architecture design trades which must consider system functionality and performance, system robustness, and fault ridethrough capability, in addition to the balance between mass and efficiency. This paper presents a quantitative comparison of the three potential candidate architectures for TeDP electrical networks. This analysis provides the foundations for establishing the feasibility of these different architectures subject to design and operational constraints. The findings of this paper conclude that a purely ac synchronous network performs best in terms of mass and efficiency, but similar levels of functionality and controllability to an architecture with electrical decoupling via dc cannot readily be achieved. If power electronic converters with cryocoolers are found to be necessary for functionality and controllability purposes, then studies show that a significant increase in the efficiency of solid-state switching components is necessary to achieve specified aircraft performance targets.

An assessment of distributed propulsion: Advanced propulsion system architectures for conventional aircraft configurations

Radical aircraft and propulsion system architecture changes may be required to continue historic performance improvement rates as current civil aircraft and engine technologies mature. Significant fuel-burn savings are predicted to be achieved through distributed propulsion, where an array of propulsors is distributed along the span of an aircraft to ingest boundary layer air and increase propulsive efficiency. The paper is divided in to two parts of which this is Part A, which presents the conceptual design tool, which has been developed to model the selected aircraft. It is followed by an evaluation of distributed propulsion on tube-and-wing aircraft configurations. The distributed propulsion aircraft has been compared with both a current technology reference aircraft and an advanced turbofan powered aircraft of 2035 technology. The latter achieved a 27.5% fuel-burn reduction relative to the baseline aircraft and the distributed propulsion variant showed further fuel efficiency gains of 4.1% due to the reduction in power required by the distributed propulsors, relative to a turbofan in free-stream conditions. Part B assesses the potential for distributed propulsion on a Blended Wing Body (BWB) aircraft configuration and compares the performance with an advanced turbofan variant BWB of the same technology level. The distributed propulsion variant BWB provided a further 5.3% fuel-burn reduction relative to the turbofan BWB through reduced core engine size and reduced specific fuel consumption.

HTS Electrical System for a Distributed Propulsion Aircraft

As the commercial aircraft industry aims to meet the challenge of 2050 emissions targets, it becomes apparent that unconventional aircraft configurations may be necessary to achieve them. Distributed electrical propulsion offers opportunities for improving aircraft efficiency. However, the conversion and transfer of power could prove prohibitive when using conventional machines and conductors due to weight and inefficiency. An aircraft with distributed propulsion and boundary layer ingestion is considered in this paper, and the current view of the future of high-temperature superconductor technology as applied to aircraft is explained.

Wing shaping concepts using distributed propulsion

Purpose – The purpose of this research is to explore innovative aircraft concepts that use flexible wings and distributed propulsion to significantly reduce fuel burn of future transport aircraft by exploiting multidisciplinary interactions. Design/methodology/approach – Multidisciplinary analysis and trajectory optimization are used to evaluate the mission performance benefits of flexible wing distributed propulsion aircraft concepts. Findings – The flexible wing distributed propulsion aircraft concept was shown to achieve a 4 per cent improvement in L/D over a mission profile consisting of a minimum fuel climb, minimum fuel cruise and continuous descent. Practical implications – The technologies being investigated may lead to mission adaptive aircraft that can minimize drag, and thus fuel burn, throughout the flight envelope. Originality/value – The aircraft concepts being explored seek to create synergistic interactions between disciplines for reducing fuel burn while capitalizing on the potential bene...

Power and protection considerations for TeDP microgrid systems

Purpose – The purpose of this paper is to highlight and discuss the unique safety and protection requirements for the electrical microgrid system in a turboelectric distributed propulsion aircraft. Design/methodology/approach – The NASA N3-X concept aircraft requirements were considered. The TeDP system was decomposed into three subsystems: turbogenerator, distribution system and propulsors. Unique considerations for each of these subsystems were identified. Findings – The fail-safe requirements for a TeDP system require a divergence from the standard safety case used for conventional propulsion systems. Advantages in flight control and single-engine-out scenarios can be realized using TeDP. Additionally, a targeted use of energy storage and reconfigurability may enable seamless response to propulsion systems failures. Practical implications – The concepts discussed in this paper will assist to guide the early conceptual and preliminary design and evaluation of TeDP architectures. Originality/value – The ...